Electrophotographic marking is a well-known method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired final image onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to produce an electrostatic latent image of the desired image on the photoreceptor's surface. Toner particles are then deposited onto that latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. By placing the various colors of toner in superimposed registration a desired composite color image results.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. That beam reflects from the facets and strikes the photoreceptor, producing a light spot. As the polygon rotates, the spot traces lines, called scan lines, on the photoreceptor. The direction of the sweeping spot is called the fast scan direction. By moving the photoreceptor perpendicular to the fast scan direction, as the polygon rotates the spot raster scans the photoreceptor. The direction of motion of the photoreceptor is referred to either as the slow scan direction or the process direction. During scanning, the laser beam is modulated to produce the desired latent image.
The number of scan lines per inch is an important measure of the quality of the final image. For example, a given printer might produce 600 scan lines per inch in the slow scan direction. Not only is the absolute number of scan lines per inch important, but so is the line spacing evenness. Errors in the slow scan direction as small as 1% of nominal line spacing may be apparent. This implies a need for a high degree of spot position control, especially in printing systems that use multiple laser beams to produce a color print.
Errors in the slow scan direction spot position arise from many sources, including polygon and/or photosensitive member motion flaws, facet and/or image plane (e.g., photosensitive medium) surface defects, etc. These errors are most commonly addressed by the optical systems within the raster output scanner. In particular, U.S. Pat. No. 5,287,125 to Appel et al. discloses a raster output scanner that has process direction (slow scan direction) spot position control that is accomplished using a piezoelectric actuator that moves a pre-polygon lens. An error feedback circuit senses the position of a moving photoreceptor. Position errors produce signals that are applied to the piezoelectric actuator. In response, the piezoelectric actuator expands or contracts, moving the pre-polygon lens, and correcting for the position errors.
While the raster output scanner disclosed in U.S. Pat. No. 5,287,125 is useful, it has its limitations. For example, connecting the pre-polygon lens to the piezoelectric actuator such that the pre-polygon lens accurately tracks the expansion and contraction of the actuator, while simultaneously providing accurate positioning of the pre-polygon lens in the focal plane direction, is difficult and expensive.
Therefore, a piezoelectric activated lens mover in which the moved lens accurately tracks the piezoelectric element and in which that lens remains accurately positioned in the focal plane would be beneficial. Even more beneficial would be a low cost, high displacement piezoelectric activated lens mover in which the moved lens accurately tracks the piezoelectric element and in which that lens remains accurately positioned in the focal plane. Particularly beneficial would be a high displacement piezoelectric activated lens mover.